1. Field of the Invention
The present invention relates, in general, to electrically enhanced air filtration and, more specifically, systems and methods for reducing air bypass between adjacent filter assemblies and between filter assemblies and adjacent surfaces of housings and/or racks in which the filter assemblies are mounted.
2. Relevant Background
Gas filtration, and more specifically air filtration, is used in a wide variety of applications ranging from automobiles, homes, office buildings and manufacturing facilities. In many cases filtration systems are used to remove pollutants such as dust, particulates, microorganisms and toxins from breathing air, although filtration systems and processes may be used to purify manufacturing environments, process gasses, combustion gasses and the like.
One particular application is for heating, ventilation, and air conditioning (HVAC) systems within buildings. HVAC systems are designed to circulate air throughout a building. HVAC systems comprise a motor and blower that moves air from a supply through ductwork that distributes the air throughout building spaces. The air supply may be outside air, re-circulated from inside the building, or a mixture of outside and re-circulated air. Toxic material entering an intake will spread quickly through the entire building. Moreover, any duct that is accessible can be used to spread contaminants. Hence, although HVAC systems contribute significantly to occupant comfort, safety, and health, and are one of the prime lines of defense against emergencies and hazards they can also quickly become a threat to the security and safety of a facility and its occupants.
Conditioning systems such as heat exchangers, humidifiers, dehumidifiers, and the like are positioned in-line with the ductwork to adjust various characteristics of the supplied air before it is delivered to building spaces. Air filtration systems are placed in-line with the ductwork to filter out particulates, organisms, odors and noxious gases from the air that are present within the flow of air. Air filtration systems used in commercial environments typically comprise a filter rack mounted inside a main duct. Small systems may include a single replaceable filter mounted in the rack. More frequently, however, the filter rack forms a grid defining multiple cells where a filter is positioned in each cell. The filter rack can be formed of extruded metal having protrusions forming integral tracks for holding filter elements. Filter elements comprise a paper or plastic frame, usually square or rectangular in shape, with filter media mounted inside the frame. A wide variety of filter media are available including loose pile fibers, woven fibers, pleated fabric, and the like. Filter elements are positioned in the tracks by sliding them in from the side (or top in unusual configurations), or by placing them in from the front or back of the rack.
In many buildings, air bypassing the filter cells is a significant problem. Air bypass means that some portion of the air is not being filtered. Air bypass can be caused by air leaking around the rack, leaking between the filter cells and the rack, old or missing gasket material, or leakage between the filter cells. Originally, the purpose of filtration in HVAC equipment was to prevent contaminants from fouling the motor/blower and cooling coils, and the like. Hence, filters were initially installed for equipment protection, not human protection. Since equipment protection mainly involved removing large particles, there was little perceived need to install a filter that was efficient at removing small particles. Filters for equipment protection are very open and therefore relatively inexpensive, since they contain relatively little filter media, and inexpensive frame material can be used as there was little concern regarding air bypass. Although such filters have a characteristically low purchase/replacement cost and low operating cost rather than improved indoor air quality, they only filter out large particulate matter such as dust, allowing smaller particulates to pass through. Air bypass has been a low-priority concern for systems in which low-cost, low-efficiency filter media already allow so much particulate matter that eliminating air bypass would provide negligible improvement. Moreover, air bypass actually reduces air resistance, and so can reduce operating energy costs.
There has been little effort to develop systems that reduce air bypass except in high-end filtration system used in, for example, electronics manufacturing, medical research facilities, and the like. In these high-end systems air bypass is specifically addressed by gel or grease seals between mating surfaces of filter elements and between the frame and filter elements. These systems work well, but are expensive to install and maintain. As a result, they are used primarily for special-purpose installations and are not practical for general-purpose commercial HVAC systems.
Filter frame elements may be formed by injection molding or similar type processes. In its simplest variation, the frame is directly formed as a unitary structure. A simple technique for this is described in U.S. Pat. No. 2,032,262 where a pleated filter is held in a jig with only the edge portions extending out of the jig. Filter frames can also be formed from multiple pieces, which are then assembled by use of adhesive, welding, friction fit, snap fits, mechanical fasteners or the like. This type of approach permits more flexibility in terms of frame forms and functionality than unitary filter frames, however, it generally results in framed filters which are complicated to manufacture while often not providing an airtight seal around the filter media. U.S. Pat. No. 6,406,509, which is incorporated herein by reference, describes a framed filter media produced using a continuous extruded frame that is notched so that the extrusion can be bent at the notches to form an enclosing frame. Each of these structures fails to prohibit air bypass between adjacent filter elements or between filter elements and the housing or filter rack in which they are positioned.
Accordingly, a need exists for filters and filter assemblies with bypass seal to reduce or prevent air bypass between adjacent filter elements or between filter elements and the housing or filter rack in which they are positioned.